1. Field of the Invention
The present invention relates to a variable gain amplifier, and more particularly to a variable gain amplifier having linear-in-decibel transconductance.
2. Description of the Related Art
An exponential gain characteristic is required in many automatic gain control (AGC) applications. In order to obtain the exponential gain characteristic, a bipolar semiconductor process and a bipolar complementary metal oxide semiconductor (BICMOS) process have been widely used. A variable gain amplifier, which has a linear-in-dB gain characteristic, manufactured by a bipolar semiconductor process, is disclosed in U.S. Pat. No. 5,999,053, which is entitled “Current Steering Variable Gain Amplifier With Linearizer”. In recent years, a CMOS process has been widely used in designing integrated circuits, thus, designing amplifiers with exponential gain characteristics using the CMOS process has also been necessary. Although a MOS transistor has no inherent exponential transmission characteristic, a circuit having linear-in-decibel gain characteristic may be implemented using a CMOS process by applying the Taylor series expansion, exp(−2x)=(1−x)/(1+x) so as to approximate the linear-in-decibel gain characteristic.